This is a better article with a map
https://www.geoengineer.org/news/casting-of-fehmarnbelt-tunn...
https://www.geoengineer.org/news/casting-of-fehmarnbelt-tunn...
This is fascinating. I had thought underwater tunnels were always creating by boring, like the Chunnel was. But this looks like they're dredging a trench, placing cast concrete segments in it, and then filling in over the top. In order words, much like how the first subway tunnels were built a century ago on dry land, but now underwater.
I wonder how it's reliably kept waterproofed (on the span of decades or centuries)? How are the segments joined and sealed?
As for longevity - it is not that different from normal tunnels. Those also need to be often heavily waterproofed due to groundwater (under quite some pressure if you are tunneling under a lot of mountains!) and various underground springs you might tunnel into. So even "regular" tunnels that are blasted or bored often first have to fill the stabilized cavity with hydro-izolation and then actually build the actual hermeticaly sealed tunnel tube inside that out of concrete.